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Title 173: Compatibility of ELM suppression with the radiating divertor in the hybrids
Name:Thomas W. Petrie () Affiliation:General Atomics
Research Area:ELM Control & Pedestal Physics Presentation time: Not requested
Co-Author(s): T. Evans, M. Fenstermacher, and M. Schaffer
Description: This experiment is the fourth in a series of experiments for determining if the ELM suppression method using the I-coil is compatible with radiating divertor scenarios in the hybrid regime. This experiment provides a side-by-side comparison of how an ELMing hybrid plasma and an ELM-free hybrid plasma with I-coil perform under puff-and-pump scenarios with argon as the injected impurity. DIII-D WOULD BE UNIQUELY CONFIGURED TO DO THIS EXPERIMENT. We focus on addressing the following questions:
* Is there a significant change in the argon accumulation in the plasma core under I-coil operation?
* How does argon entrainment in the divertor change when the I-coil is activated?
* How does the ratio of divertor-to-core radiated power change when the I-coil is activated?

This experiment will be attempted only after it has been demonstrated that RMP ELM suppression has been demonstrated in the hybrid regime, where the gradB ion drift is directed OUT of the divertor.
Experimental Approach/Plan: The plasma is maintained in a ELMing hybrid H-mode regime (e.g., Ip =1.2 MA, Bt = -1.7 T, dRsep = +1.0 cm). This setup was shown previously (EPS2005) to be robust under puff and pump operation (examples: 123265-123279). Argon is injected into the private flux region of the upper divertor, while deuterium plasma flow toward the divertor is enhanced by a combination of D2 gas injected from upstream of the upper divertor targets and active cryo-pumping from both upper divertor locations. A radiating divertor plasma in the ELMing hybrid regime is established previous to t = 4.5 s of the discharge. At t = 4.5 s, the I-coil is activated and the ELMs are eliminated. This provides a side-by-side comparison between ELMing hybrid and ELM-free hybrid.

The argon injection rate and the deuterium injection rate are separately scanned. Key measurables are the changes in the radiated power distribution and heat flux values, the accumulation of argon in the core and divertor plasmas, and the density and temperature conditions at both divertor targets.
Background: Plasma operation in the hybrid regime has been put forward as a paradigm for ITER, and would become particularly attractive if ELMing could be eliminated. Suppressing ELMs from hybrid H-mode plasmas using the I-coil approach is an intriguing possibility for resolving the ELM-issue in ITER. However, even if the damage to the divertor structure from ELMs pulses were eliminated via the I-coil approach, steady peak power loading at the divertor targets could still be unacceptably high. A radiating divertor solution, in this case, puff and pump, has shown promise as effective way of reducing peak power loading in ELMing hybrid plasmas, while at the same time maintaining good hybrid properties, e.g., energy confinement time. Our experiment will address whether the puff and pump radiating divertor concept is also as effective for hybrid ELM-free plasmas during I-coil operation. At this juncture, the I-coil method has not been completely successful in eliminating ELMs for plasmas in the hybrid regime. Clearly, this experiment should be attempted after ELM suppression in hybrid is successfully demonstrated.
Resource Requirements: Machine time: 0.5-1.0 day, I-coil, dome- and upper baffle cryo-pumps cold, minimum 6 beams.
Diagnostic Requirements: Asdex gauges (in particular, dome and upper baffle locations), Penning gauge, core Thomson scattering, upper divertor and centerpost fixed Langmuir probes, and CER.
Analysis Requirements: UEDGE, with some ONETWO
Other Requirements: I-coil